1. Field of the Invention
The present invention relates to a distributed optical amplifier and a distributed optical amplifying method for adjusting an incident power of the light to be amplified depending on the optical power of pump light, considering the non-linear optical effects generated in the light to be amplified.
Moreover, the present invention relates to an optical communication system that can improve the transmission characteristic by utilizing such distributed optical amplifier.
An ultra-long distance and large capacity optical communication apparatus is now requested, aiming at establishment of future multimedia network. Development and research are continued for wavelength- division multiplexing (hereinafter abbreviated as xe2x80x9cWDMxe2x80x9d) as the system to realize large capacity transmission system from the advantageous viewpoint of wide frequency band of optical fiber and effective use of large capacity.
Particularly, in the ultra-long distance optical communication system, since the WDM optical signal is attenuated during transmission through the optical transmission line, the WDM signal must be amplified.
2. Description of the Related Art
The optical communication system of the related art comprises a transmitting terminal for generating the WDM optical signal formed by wavelength-division multiplexing of a plurality of optical signals of different wavelengths, an optical transmission line for transmitting the WDM optical signal transmitted from the transmitting terminal and a receiving terminal for receiving the transmitted WDM optical signal and moreover this optical communication system also comprises, as required, one or a plurality of repeaters having the function to amplify the WDM optical signal in the course of the optical transmission line.
In such optical communication system, waveform of each optical signal is deteriorated due to the non-linear optical effects in the optical transmission line. In order to eliminate deterioration of waveform, it is effective to reduce the optical power (optical intensity) of the WDM optical signal incident to the optical transmission line, but reduction of optical power results in deterioration of signal to noise ratio (hereinafter, referred to as xe2x80x9coptical SNRxe2x80x9d). As the non-linear optical effects, for example, self phase modulation (hereinafter, referred to as xe2x80x9cSPMxe2x80x9d), cross-phase modulation (hereinafter, referred to as xe2x80x9cXPMxe2x80x9d), four-wave mixing (hereinafter, referred to as xe2x80x9cFWMxe2x80x9d), stimulated Raman scattering (hereinafter, referred to as xe2x80x9cSRSxe2x80x9d) and stimulated Brillouin scattering (hereinafter, referred to as xe2x80x9cSBSxe2x80x9d) are known.
For this purpose, it has been proposed to use in combination a centralized optical amplifier provided within a repeater and a distributed optical amplifier using the optical transmission line in common as the optical amplifying medium. For example, effectiveness of the Raman amplification is reported in P. B. Hansen, A. Stentz, T. N. Nielsen, R. Espinodola, L. E. Nelson, A. A. Abramov, xe2x80x9cDense wavelength-division multiplexed transmission in xe2x80x9czero-dispersionxe2x80x9d DSF by means of hybrid Raman/erbium-doped fiber amplifierxe2x80x9d (OFC/100C ""99), PD8, 19999 and N. Takachio, H. Suzuki,. H. Masuda and M. Koga xe2x80x9c32*10 Gb/s distributed Raman amplification transmission with 50-GHz channel spacing in the zero-dispersion region over 640 km of 1.55-xcexcm, dispersion-shifted fiberxe2x80x9d (OFC/100C ""99), PD9, 1999.
Moreover, the Japanese Published Unexamined Patent Application No. HEI 03-013836 (Japanese Patent Application No. HEI 01-149148) discloses a method of Raman amplification by obtaining loss of the optical transmission line through detection of rear scattering of the incident test light to the optical transmission line.
The Japanese Published Unexamined Patent Application No. HEI 10-073852 (Japanese Patent Application No. HEI 08-232376) discloses the Raman amplification in the widened amplification band using a plurality of pump lights in different wavelengths.
The Japanese Published Unexamined Patent Application No. HEI 10-073852 (Japanese Patent Application No. HEI 08-170183) discloses inclusion of a pump light source for Raman amplification within a repeater.
Here, an optical amplifier may be classified into a centralized optical amplifier and a distributed optical amplifier. The centralized optical amplifier is an optical amplifier wherein an optical amplifying medium and a pump light source are centralized in one area. For example, a semiconductor laser amplifier and an optical fiber amplifier that is formed by winding an optical fiber as an amplifying medium around a bobbin are well known. On the other hand, the distributed optical amplifier is an optical amplifier wherein an optical amplifying medium is laid for a constant distance and the pump light source is provided in one or both areas. For example, an optical fiber amplifier is proposed. As an optical fiber amplifier, rare-earth element added optical fiber amplifier and an optical fiber amplifier utilizing the non-linear scattering in the optical fiber are proposed.
These centralized optical amplifier and distributed optical amplifier are identical in the physical process to amplify the optical signal but are mainly different in such a point that the optical amplifying medium is summarized within one area or is distributed for a constant distance. The distributed optical amplifier is characterized in that the optical amplifying medium can also be used as the inter-terminal optical transmission line for transmitting the optical signal.
Moreover, as the non-linear scattering, SRS and SBS are known. SRS is the scattering generated due to the mutual effect of optical phonon of lattice vibration and has wide gain width and large frequency shift. While, SBS is the scattering generated due to the mutual effect of acoustic phonon of lattice vibration and has gain width narrower than that of SRS and small frequency shift but has the gain efficient larger than that by two digits or more.
The optical fiber amplifier using the non-linear scattering is characterized in that an ordinary optical fiber such as NZ-DSF and SMF can be used, the pumping wavelength can be set for any amplification wavelength and gain is matched in the polarizing direction of the pump light. As the ordinary optical fibers, for example, distributed shift fiber (hereinafter, abbreviated as xe2x80x9cDSFxe2x80x9d), non-zero distributed shift optical fiber (hereinafter, abbreviated as xe2x80x9cNZ-DSFxe2x80x9d), distributed flat optical fiber (hereinafter, abbreviated as xe2x80x9cDFFxe2x80x9d) and 1.3 xcexcm zero-distributed (normal distribution) single mode optical fiber (hereinafter, abbreviated as xe2x80x9cSMFxe2x80x9d) are proposed.
The references cited above disclose the Raman amplification but does not disclose the practical method how to control the optical power of the pump light and the optical power of light to be amplified with the distributed optical amplifier. Otherwise, if such reference discloses the practical method for such control, such reference requires a complicated circuit for the control of optical power.
An object the present invention is to provide a distributed optical amplifier that can control the optical power of pump light and the optical power of the light to be amplified with a simplified structure.
Another object of the present invention is to provide an optical power control method that is just suitable for this distributed optical amplifier.
Still another object of the present invention is to provide an optical communication system utilizing this distributed optical amplifier.
The above objects can be accomplished by a distributed optical amplifier comprising: optical amplifying medium for distributed optical amplification; light supplying part for supplying a pump light to the optical amplifying medium; a pump light detecting part for detecting optical power of the pump light; adjusting part for adjusting an incident power of the light amplified with the optical amplifying medium; and control part for adjusting incident optical power of the light according to an output of the pump light detecting part.
A control part of distributed optical amplifier as such controls the incident optical power according to a correspondence relationship between the incident optical power and the output of the pump light detecting part, or controls the exit optical power at the incident end of the pump light to less than the optical power that generates a predetermined waveform deterioration with the non-linear optical effects depending on the optical amplifying medium, when the light is incident under the condition that the pump light is supplied to the optical amplifying medium. A distributed optical amplifier as such may, for example, use an optical transmission line as an optical amplifying medium, or supply a pump light from a plurality of parts. Also, a distributed optical amplifier as such may, for example, further comprise a residual pump light detecting part for detecting optical power of residual pump light or an amplified light detecting part for detecting optical power of amplified light.
Also, the above objects may be accomplished by an optical communication system for transmitting an optical signal between a couple of terminals, comprising: an optical transmission line for transmitting the optical signal along with performing distribution amplifying; light supplying part for supplying a pump light to the optical transmission line; a pump light detecting part for detecting optical power of the pump light; adjusting part for adjusting an incident power of the light amplified with the optical amplifying medium; and control part for adjusting incident optical power of the light according to an output of the pump light detecting part.
Moreover, the above object may be accomplished by an optical communication system, comprising: a transmitting terminal for outputting wavelength-multiplexed optical signal; an optical transmission line for inputting an output light from the transmitting terminal; and pumping part for inserting a pump light for pumping the optical transmission line to the output end side of the optical transmission line to perform stimulated Raman amplification in the optical transmission line. An optical system as such may, for example, set the optical power of the optical signal output from the transmitting terminal to a value less than a value that actually generates crosstalk by four-wave mixing at the output end of the optical transmission line, or, to a value less than a value that actually generates the predetermined waveform deterioration due to cross-phase modulation at the output end of the optical transmission line. Moreover, in cases like these, stimulated Brillouin scattering and signal to noise ratio are considered.
In such distributed optical amplifier and optical communication system, the correspondence relationship of incident optical power for the pump light is previously determined so that the light to be amplified should not generate the non-linear optical effects higher than the predetermined range in the optical amplifying medium. The control part adjusts the incident optical power with the adjusting part according to the correspondence relationship depending on an output of the pump light detecting part for detecting the excited optical power. Therefore, when the optical signal is amplified with such distributed optical amplifier and optical communication system, the optical signal in which waveform deterioration is controlled within the predetermined design range. When the light is amplified again with the centralized optical amplifier after it is amplified first with the distributed optical amplifier, an optical signal of large optical power can be incident to the centralized optical amplifier. Therefore, the total optical SNR in the distributed optical amplifier and centralized optical amplifier can be much improved. This way, this optical transmission system makes ultra-long distance communication possible.
Such distributed optical amplifier can control the waveform deterioration and optical SNR with the simplified structure of the pump light detecting part, adjusting part and control part.